A counterfeit is an imitation that is made with the intent to deceptively represent its content or origins. The word counterfeit most frequently describes forged money or documents, but can also describe clothing, software, pharmaceuticals, or any other manufactured item.

Traditionally, anti-counterfeiting measures involved including fine detail on bills which would allow non-experts to easily spot forgeries. On coins, milled or reeded (marked with parallel grooves) edges are used to show that none of the valuable metal has been scraped off. This detects the shaving or clipping (paring off) of the rim of the coin. However, it does not detect sweating, or shaking coins in a bag and to collect the resultant dust. Since this technique removes a smaller amount, it is primarily used on the most valuable coins, primarily gold.

In the late twentieth century advances in computer and photocopy technology made it possible for people without sophisticated training to easily copy currency. In response, national engraving bureaus began to include new more sophisticated anti-counterfeiting systems such as holograms, and inks whose colors changed depending on the angle of the light.